Well system with degradable plug

ABSTRACT

A downhole assembly is disclosed. The downhole assembly includes a tube disposed in a wellbore, and a shroud coupled to and disposed around the circumference of the tube to form an annulus between an inner surface of the shroud and an outer surface of the tube. The downhole assembly further includes a flow control device disposed in the annulus, and a degradable plug disposed in the annulus and positioned to prevent fluid flow between the annulus and the tube.

TECHNICAL FIELD

The present disclosure is related to downhole tools for use in awellbore environment and more particularly to degradable plugs used totemporarily block fluid flow in a well system.

BACKGROUND OF THE DISCLOSURE

After a wellbore has been formed for the purpose of exploration orextraction of natural resources such as hydrocarbons or water, variousdownhole tools may be inserted into the wellbore to extract the naturalresources from the wellbore and/or to maintain the wellbore. At varioustimes during production and/or maintenance operations, it may benecessary to temporarily block the flow of fluid into or out of variousportions of the wellbore or various portions of the downhole tools usedin the wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete and thorough understanding of the various embodimentsand advantages thereof may be acquired by referring to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numbers indicate like features, and wherein:

FIG. 1 is an elevation view of a well system;

FIG. 2 is a cross-sectional view of a downhole assembly including adegradable plug in-line with and adjacent to a flow control device;

FIG. 3 is a cross-sectional view of a downhole assembly including adegradable plug in-line with and axially displaced from a flow controldevice;

FIG. 4 is a cross-sectional view of a downhole assembly including adegradable plug axially and radially displaced from a flow controldevice;

FIG. 5A is a cross-sectional view of a degradable plug including ano-ring seal;

FIG. 5B is a cross-sectional view of a press-fit degradable plug;

FIG. 5C is a cross-sectional view of a tapered press-fit degradableplug;

FIG. 5D is a cross-sectional view of a threaded degradable plug;

FIG. 5E is a cross-sectional view of a swage-fit degradable plug;

FIG. 6A is a cross-sectional view of a degradable plug formed of adegradable composition that is reactive under defined conditions;

FIG. 6B is a cross-sectional view of a degradable plug including a shelland a core disposed within the shell and formed of a degradablecomposition that is reactive under defined conditions;

FIG. 6C is a cross-sectional view of a degradable plug including ashell, a core disposed within the shell and formed of a degradablecomposition that is reactive under defined conditions, and a rupturedisk;

FIG. 6D is a cross-sectional view of a degradable plug including a coreformed of a degradable composition that is reactive under definedconditions and disposed within a shell including a diffusion channel;and

FIG. 7 is a flow-chart of a method of temporarily preventing the flow ofproduction fluids into a production string.

DETAILED DESCRIPTION OF THE DISCLOSURE

Embodiments of the present disclosure and its advantages may beunderstood by referring to FIGS. 1 through 7, where like numbers areused to indicate like and corresponding parts.

Production fluids, including hydrocarbons, water, sediment, and othermaterials or substances found in a formation may flow from the formationinto a wellbore through the sidewalls of the open hole portions of thewellbore. The production fluids may circulate in the wellbore beforebeing extracted via a downhole assembly. The downhole assembly mayinclude a screen to filter sediment from the production fluids flowinginto the downhole assembly and a flow control device to regulate theflow of production fluids into the downhole assembly. Similarly,injection fluids may flow from a production string into the downholeassembly before flowing into the wellbore. A plug may be used totemporarily prevent flow of production or injection fluids between thedownhole assembly and the wellbore. The plug may be positioned axiallywith respect to the flow control device. To resume fluid flow betweenthe downhole assembly and the wellbore, the plug may be removed. Toavoid the cost and time associated with manual removal of the plug, itmay be removed via a chemical reaction that causes the plug to degradewithin the wellbore.

FIG. 1 is an elevation view of an example embodiment of a well system.Well system 100 may include well surface or well site 106. Various typesof equipment such as a rotary table, drilling fluid or production fluidpumps, drilling fluid tanks (not expressly shown), and other drilling orproduction equipment may be located at well surface or well site 106.For example, well site 106 may include drilling rig 102 that may havevarious characteristics and features associated with a “land drillingrig.” However, downhole drilling tools incorporating teachings of thepresent disclosure may be satisfactorily used with drilling equipmentlocated on offshore platforms, drill ships, semi-submersibles anddrilling barges (not expressly shown).

Well system 100 may also include production string 103, which may beused to produce hydrocarbons such as oil and gas and other naturalresources such as water from formation 112 via wellbore 114.Alternatively, or additionally, production string 103 may be used toinject hydrocarbons such as oil and gas and other natural resources suchas water into formation 112 via wellbore 114. As shown in FIG. 1,wellbore 114 is substantially vertical (e.g., substantiallyperpendicular to the surface). In other embodiments, portions ofwellbore 114 may be substantially horizontal (e.g., substantiallyparallel to the surface), or at an angle between vertical andhorizontal. Casing string 110 may be placed in wellbore 114 and held inplace by cement, which may be injected between casing string 110 and thesidewalls of wellbore 114. Casing string 110 may provide radial supportto wellbore 114 and may seal against unwanted communication of fluidsbetween wellbore 114 and surrounding formation 112. Casting string 110may extend from well surface 106 to a selected downhole location withinwellbore 114. Portions of wellbore 114 that do not include casing string110 may be described as “open hole.”

The terms “uphole” and “downhole” may be used to describe the locationof various components relative to the bottom or end of wellbore 114shown in FIG. 1. For example, a first component described as uphole froma second component may be further away from the end of wellbore 114 thanthe second component. Similarly, a first component described as beingdownhole from a second component may be located closer to the end ofwellbore 114 than the second component.

Well system 100 may also include downhole assembly 120 coupled toproduction string 103. Downhole assembly 120 may be used to performoperations relating to the completion of wellbore 114, the production ofhydrocarbons and other natural resources from formation 112 via wellbore114, the injection of hydrocarbons and other natural resources intoformation 112 via wellbore 114, and/or the maintenance of wellbore 114.Downhole assembly 120 may be located at the end of wellbore 114 or at apoint uphole from the end of wellbore 114. Downhole assembly 120 may beformed from a wide variety of components configured to perform theseoperations. For example, components 122 a, 122 b and 122 c of downholeassembly 120 may include, but are not limited to, screens, flow controldevices, slotted tubing, packers, valves, sensors, and actuators. Thenumber and types of components 122 included in downhole assembly 120 maydepend on the type of wellbore, the operations being performed in thewellbore, and anticipated wellbore conditions.

Production fluids, including hydrocarbons, water, sediment, and othermaterials or substances found in formation 112 may flow from formation112 into wellbore 114 through the sidewalls of the open hole portions ofwellbore 114. The production fluids may circulate in wellbore 114 beforebeing extracted via production string 103. Alternatively, oradditionally, injection fluids, including hydrocarbons, water, and othermaterials, may be injected into wellbore 114 and formation 112 viaproduction string 103 and downhole assembly 120. Downhole assembly 120may include a screen (shown in FIG. 2) to filter sediment fromproduction fluids flowing into production string 103. Downhole assembly120 may also include a flow control device to regulate the flow ofproduction fluids into production string 103. Downhole assembly 120 mayalso include a plug that may be used to temporarily prevent flow ofproduction fluids into production string 103 or injection fluids out ofproduction string 103. To avoid the cost and time associated with manualremoval of the plug, it may be removed via a chemical reaction thatcauses the plug to degrade within wellbore 114.

FIG. 2 is a cross-sectional view of a downhole assembly including adegradable plug in-line with and adjacent to a flow control device.Production fluids circulating in wellbore 114 may flow through downholeassembly 200 into production string 103. Downhole assembly 200 may belocated downhole from production string 103 and may be coupled toproduction string via tubing 210. In some embodiments, downhole assembly200 may be coupled to production string 103 by a threaded joint. Inother embodiments, a different coupling mechanism may be employed. Thecoupling of downhole assembly 200 and production string 103 may alsoprovide a fluid and pressure tight seal.

Downhole assembly 200 may include screen 202 and shroud 204, which maybe coupled to and disposed downhole from screen 202. Both screen 202 andshroud 204 may be coupled to and disposed around the circumference oftubing 210 such that annulus 212 is formed between the inner surfaces ofscreen 202 and shroud 204 and the outer surface of tubing 210.Production fluids circulating in wellbore 114 may enter downholeassembly 200 by flowing through screen 202 into annulus 212. Screen 202may be configured to filter sediment from production fluids as they flowthrough screen 202. Screen 202 may include, but is not limited to, asand screen, a gravel filter, a mesh, or slotted tubing.

Downhole assembly 200 may also include flow control device 206 disposedwithin annulus 212 between shroud 204 and tubing 210. Flow controldevice 206 may include channel 214 extending there through to permit theflow of production fluids through flow control device 206. Flow controldevice 206 may engage with shroud 204 and tubing 210 to preventproduction fluids circulating in annulus 212 from flowing between flowcontrol device 206 and tubing 210 or shroud 204. For example, flowcontrol device 206 may engage with the inner surface of shroud 204 toform a fluid and pressure tight seal and may engage with the outersurface of tubing 210 to form a fluid and pressure tight seal. Becauseflow control device 206 engages with tubing 210 and shroud 204 to form afluid and pressure tight seal, production fluids circulating in annulus212 flow through channel 214 rather than between flow control device 206and tubing 210 or between flow control device 206 and shroud 204.

The flow of production fluids through channel 214 may be temporarilyblocked by plug 208 disposed in a portion of annulus 212 downhole fromflow control device 206. Plug 208 may be positioned in-line with andadjacent to flow control device 206, as shown in FIG. 2. Plug 208 mayengage with shroud 204 and tubing 210 to form a fluid and pressure tightseal, thereby preventing production fluids from flowing into the portionof annulus downhole from flow control device 206. Plug 208 may also beused to temporarily block the flow of injection fluids from productionstring 103 into wellbore 114 and formation 112. For example, the flow ofinjection fluids from production string into wellbore 114 and formation112 may be temporarily blocked by plug 208 positioned in-line with andadjacent to flow control device 206, as shown in FIG. 2. Plug 208 mayengage with shroud 204 and tubing 210 to form a fluid and pressure tightseal, thereby preventing injection fluids from flowing into the portionof annulus uphole from flow control device 206.

Plug 208 may be formed of a degradable composition including a metal oralloy that is reactive under defined conditions. Plug 208 may be removedfrom annulus 212 using a chemical reaction that causes plug 208 todegrade, thereby avoiding manual intervention required to extract plug208 from annulus 212 using a retrieval tool. The term “degrade” may beused to describe a process by which a component breaks down into piecesor dissolves into particles small enough that they do not impede theflow of fluids. The features of plug 208, including its degradability,are described in additional detail with respect to FIGS. 5A-5E and6A-6D. Once the chemical reaction causing plug 208 to degrade has beentriggered, the reaction may continue until plug 208 breaks down intopieces or dissolves into particles small enough that they do not impedethe flow of production fluids through channel 214 of flow control device206. When plug 208 has degraded to this point, production fluids mayflow through channel 214 of flow control device 206 and into the portionof annulus 212 downhole from flow control device 206. From there, theproduction fluids may flow through opening 216 formed in a sidewall oftubing 210 into tubing 210 and into production string 103.

Downhole assembly 200 may also include port 218, which may be removed topermit access to the portion of annulus 212 downhole from flow controldevice 206. Port 218 may be coupled to shroud 204 and tubing 210 via athreaded connection. Port 218 may engage with shroud 204 and tubing 210to form a fluid and pressure tight seal. Port 218 may include a socketor slot into which a tool may be inserted. With a tool inserted into thesocket or slot, port 218 may be rotated in order to disengage thethreaded connection between port 218 and 204. When port 218 has beenremoved, plug 208 may be replaced (i.e., a new plug may be installed).For example, after plug 208 has been removed via a chemical reactioncausing plug 208 to degrade, the flow of production fluids throughchannel 214 of flow control device 206 may again be temporarily blockedby replacing plug 208.

FIG. 3 is a cross-sectional view of a downhole assembly including adegradable plug in-line with and axially displaced from a flow controldevice. Production fluids circulating in wellbore 114 may enter downholeassembly 200 by flowing through screen 202 into annulus 212. Productionfluids may then flow through channel 214 of flow control device 206 intothe portion of annulus 212 downhole from flow control device 206.Production fluids may be temporarily blocked from flowing throughopening 216 into tubing 210 and production string 103 by plug 208disposed in the portion of annulus 212 downhole from flow control device206. Plug 208 may be positioned in-line with and axially displaced fromflow control device 206, as shown in FIG. 3. Plug 208 may engage withshroud 204 and tubing 210 to form a fluid and pressure tight seal,thereby preventing production fluids from flowing into the portion ofannulus downhole from plug 208.

Plug 208 may also be used to temporarily block the flow of injectionfluids from production string 103 into wellbore 114 and formation 112.For example, the flow of injection fluids from production string intowellbore 114 and formation 112 may be temporarily blocked by plug 208positioned in-line with and axially displaced from flow control device206, as shown in FIG. 3. Plug 208 may engage with shroud 204 and tubing210 to form a fluid and pressure tight seal, thereby preventinginjection fluids from flowing into the portion of annulus uphole fromflow control device 206.

As explained above with respect to FIG. 2, plug 208 may be formed of adegradable composition including a metal or alloy that is reactive underdefined conditions. Plug 208 may be removed from annulus 212 using achemical reaction that causes plug 208 to degrade, thereby avoidingmanual intervention required to extract plug 208 from annulus 212 usinga retrieval tool. Once the chemical reaction causing plug 208 to degradehas been triggered, the reaction may continue until plug 208 breaks downinto pieces or dissolves into particles small enough that they do notimpede the flow of production fluids through annulus 212 or opening 216.When plug 208 has degraded to this point, production fluids may flowthrough opening 216 into tubing 210 and into the production string 103.

FIG. 4 is a cross-sectional view of a downhole assembly including adegradable plug axially and radially displaced from a flow controldevice. Production fluids circulating in wellbore 114 may enter downholeassembly 200 by flowing through screen 202 into annulus 212. Productionfluids may then flow through channel 214 of flow control device 206 intothe portion of annulus 212 downhole from flow control device 206.Production fluids may be temporarily blocked from flowing throughopening 216 into tubing 210 and production string 103 by plug 208. Plug208 may be positioned within opening 216 and may engage with opening 216to form a fluid and pressure tight seal, thereby preventing productionfluids from flowing between annulus 212 and tubing 210. Plug 208 mayalso be used to temporarily block the flow of injection fluids fromproduction string 103 into wellbore 114 and formation 112. For example,the flow of injection fluids from production string into wellbore 114and formation 112 may be temporarily blocked by plug 208 positionedwithin opening 216, as shown in FIG. 4. Plug 208 may engage with opening216 to form a fluid and pressure tight seal, thereby preventinginjection fluids from flowing between annulus 212 and tubing 210.

As explained above with respect to FIG. 2, plug 208 may be formed of adegradable composition including a metal or alloy that is reactive underdefined conditions. Plug 208 may be removed from opening 216 using achemical reaction that causes plug 208 to degrade, thereby avoidingmanual intervention required to extract plug 208 from opening 216 usinga retrieval tool. Once the chemical reaction causing plug 208 to degradehas been triggered, the reaction may continue until plug 208 breaks downinto pieces or dissolves into particles small enough that they do notimpede the flow of production fluids through opening 216. When plug 208has degraded to this point, production fluids may flow through opening216 into tubing 210 and into the production string 103.

A variety of mechanisms may be employed to permit plug 208 to form afluid and pressure tight seal with shroud 204 and tubing 210 (asdiscussed with respect to FIGS. 2 and 3) or with opening 216 (asdiscussed with respect to FIG. 4). FIGS. 5A-5E illustrate exemplarymechanisms that may be used to form a fluid and pressure tight sealbetween plug 208 and shroud 204 and tubing 210 (as discussed withrespect to FIGS. 2 and 3) or opening 216 (as discussed with respect toFIG. 4).

FIG. 5A is a cross-sectional view of a degradable plug including ano-ring seal. Plug 208 may include seal 502 disposed around thecircumference of plug 208. Seal 502 may be inset into a groove on thesurface of plug 208 (as shown in FIG. 5A) or may be disposed on thesurface of plug 208. Although one seal 502 is depicted in FIG. 5A, anynumber of seals 502 may be used. Seal 502 may be a molded seal made ofan elastomeric material. The elastomeric material may be formed ofcompounds including, but not limited to, natural rubber, nitrile rubber,hydrogenated nitrile, urethane, polyurethane, fluorocarbon,perflurocarbon, propylene, neoprene, hydrin, etc. The elastomericmaterial may also be a degradable elastomeric material. Examples ofdegradable elastomeric material include but are not limited to EPDMrubber, natural rubber, elastomers containing polyglocolic acid,elastomers containing polylactic acid, or elastomers containing thiol.Seal 502 may engage with shroud 204 and tubing 210 form a fluid andpressure tight seal.

Although plug 208 is shown in FIG. 5A positioned in-line with andadjacent to flow control device 206, plug 208 may also be positionedin-line with and axially displaced from flow control device 206 (asshown in FIG. 3) or within opening 216 (as shown in FIG. 4). Where plug208 is positioned as shown in FIG. 3, seal 502 may engage with shroud204 and tubing 210 to form a fluid and pressure tight seal. Where plug208 is positioned as shown in FIG. 4, seal 502 may engage with opening216 to form a fluid and pressure tight seal.

FIG. 5B is a cross-sectional view of a press-fit degradable plug. Plug208 may include protrusions 504 extending radially from the surface ofplug 208. The distance that protrusions 504 extend from the surface ofplug 208 may be chosen to provide an interference fit betweenprotrusions 504 and the surface with which they are sealing. Forexample, protrusions 504 may extend radially from the surface of plug208 to provide an interference fit with shroud 504 and tubing 210. Theinterference fit between protrusions 504 and shroud 204 and betweenprotrusions 504 and tubing 210 may provide a fluid and pressure tightseal.

Although plug 208 is shown in FIG. 5B positioned in-line with andadjacent to flow control device 206, plug 208 may also be positionedin-line with and axially displaced from flow control device 206 (asshown in FIG. 3) or within opening 216 (as shown in FIG. 4). Where plug208 is positioned as shown in FIG. 3, the interference fit betweenprotrusions 504 and shroud 204 and between protrusions 504 and tubing210 may provide a fluid and pressure tight seal. Where plug 208 ispositioned as shown in FIG. 4, protrusions 504 may extend radially fromthe surface of plug 208 to provide an interference fit with opening 216.The interference fit between protrusions 504 and opening 216 may providea fluid and pressure tight seal.

FIG. 5C is a cross-sectional view of a press-fit degradable plug. Plug208 may include tapered end 506. Tapered end 506 of plug 208 may extendpartially into channel 214 of flow control device 206. Tapered end 506may be configured to provide an interference fit between plug 208 andflow control device 206. The interference fit between tapered end 506and flow control device 206 may provide a fluid and pressure tight seal.Although plug 208 is depicted in FIG. 5C positioned in-line with andadjacent flow control device 206, plug 208 may also be positioned withinopening 216 (as shown in FIG. 4). Where plug 208 is positioned as shownin FIG. 4, tapered end 506 may extend partially into opening 216.Tapered end 506 may be configured to provide an interference fit betweenplug 208 and opening 216. The interference fit between plug 208 andopening 216 may provide a fluid and pressure tight seal.

FIG. 5D is a cross-sectional view of a threaded degradable plug. Plug208 may include threads 508 configured to engage with threads 510 ofshroud 204 and threads 512 of tubing 210. The engagement of threads 508with threads 510 and threads 512 may provide a fluid and pressure tightseal. Although plug 208 is depicted in FIG. 5D positioned in-line withand adjacent flow control device 206, plug 208 may also be positionedin-line with and axially displaced from flow control device 206 (asshown in FIG. 3) or within opening 216 (as shown in FIG. 4). Where plug208 is positioned as shown in FIG. 3, the engagement of threads 508 withthreads 510 and threads 512 may provide a fluid and pressure tight seal.Where plug 208 is positioned as shown in FIG. 4, threads 508 may beconfigured to engage with threads formed on the surface of opening 216.The engagement of threads 508 with threads formed on the surface ofopening 216 may provide a fluid and pressure tight seal. A sealant maybe applied to or disposed within the threads to enhance the seal.

FIG. 5E is a cross-sectional view of a swage-fit degradable plug. Plug208 may be configured to engage with swage fitting 514 to provide aninterference fit between plug 208 and swage fitting 514. Plug 208 may beshrink-fit into swage fitting 514. The interference fit between plug 208and swage fitting 514 may provide a fluid and pressure tight seal.Although plug 208 and swage fitting 514 are depicted in FIG. 5Dpositioned adjacent flow control device 206, plug 208 and swage fitting514 may also be positioned in-line with and axially displaced from flowcontrol device 206 (as shown in FIG. 3). Additionally, plug 208 andswage fitting 514 may be positioned within opening 216 (as shown in FIG.4).

FIGS. 6A-6D illustrate exemplary embodiments of a degradable plug. FIG.6A is a cross-sectional view of a degradable plug formed of degradablecomposition that is reactive under defined conditions. Plug 208 mayinclude socket 602 which may be configured to engage with a tool topermit plug 208 to be positioned within or extracted from downholeassembly 200 (shown in FIG. 2). As discussed above with respect to FIG.2, plug 208 may be formed of a degradable composition including a metalor alloy that is reactive under defined conditions. The composition ofplug 208 may be selected such that plug 208 begins to degrade within apredetermined time of first exposure to a corrosive or acidic fluid dueto reaction of the metal or alloy from which plug 208 is formed with thecorrosive or acidic fluid. Additionally, the composition of plug 208 maybe selected such that the degradation of plug 208 accelerates withincreasing salinity or with decreasing pH of the corrosive or acidicfluid. The composition of plug 208 may further be selected such thatplug 208 degrades sufficiently to form pieces or particles small enoughthat they do not impede the flow of production fluids through channel214 of flow control device 206 (shown in FIG. 2) or opening 216 (shownin FIG. 2). The corrosive or acidic fluid may already be present withinannulus 212 (shown in FIG. 2) during operation of wellbore 114 (shown inFIG. 1) or may be injected into annulus 212 to trigger a chemicalreaction that causes plug 208 to degrade. Additionally, the fluid may beintroduced as part of the wellbore cleanup procedures. Examples ofcorrosive or acidic fluids include organic acids and inorganic acids,such as hydrochloric acid, acetic acid, citric acid, carbonic acid,lactic acid, glycolic acid, and hydrofluoric acid. Exemplarycompositions from which plug 208 may be formed include compositions inwhich the metal or alloy is selected from one of calcium, magnesium,aluminum, and combinations thereof. The composition of plug 208 may beformed from a solution process, from a powder metallurgy process, orfrom a nanomatrix composite. Additionally or alternatively, thecomposition of plug 208 may be cast, extruded, or forged. Thecomposition of plug 208 may also be heat treated or annealed.

Plug 208 may also be formed from the metal or alloy imbedded with smallparticles (e.g., particulates, powders, flakes, fibers, and the like) ofa non-reactive material. The non-reactive material may be selected suchthat it remains structurally intact even when exposed to the corrosiveor acidic fluid for a duration of time sufficient to degrade the metalor alloy into pieces or particles small enough that they do not impedethe flow of production fluids through channel 214 of flow control device206 (shown in FIG. 2) or opening 216 (shown in FIG. 2). When the metalor alloy degrades, the small particles of the non-reactive material mayremain. The particle size of the non-reactive material may be selectedsuch that the particles are small enough that they do not impede theflow of production fluids through channel 214 of flow control device 206(shown in FIG. 2) or opening 216 (shown in FIG. 2). The non-reactivematerial may be selected from one of lithium, bismuth, calcium,magnesium, and aluminum (including aluminum alloys) if not alreadyselected as the reactive metal or alloy, and combinations thereof.

Plug 208 may also be formed from the metal or alloy imbedded with smallparticles (e.g., particulates, powders, flakes, fibers, and the like) toform a galvanic cell. The composition of the particles may be selectedsuch that the metal from which the particles are formed has a differentgalvanic potential than the metal or alloy in which the particles areimbedded. Contact between the particles and the metal or alloy in whichthey are imbedded may trigger microgalvanic corrosion that causes plug208 to degrade. Exemplary compositions from which the particles may beformed include iron, steel, aluminum alloy, zinc, magnesium, graphite,nickel, copper, carbon, tungsten, and combinations thereof.

Plug 208 may also be formed from an anodic material imbedded with smallparticles of cathodic material. The anodic and cathodic materials may beselected such that plug 208 begins to degrade upon exposure to a brinefluid, which may also be referred to as an electrolytic fluid, due to anelectrochemical reaction that causes the plug to corrode. A brine fluidor electrolytic fluid may include fluids containing NaCL, KCL, and othersalts. Exemplary compositions from which the anodic material may beformed include one of magnesium, aluminum, and combinations thereof.Exemplary compositions from which the cathodic material may be formedinclude one of iron, nickel, copper, graphite, tungsten, andcombinations thereof. The anodic and cathodic materials may be selectedsuch that plug 208 is degraded sufficiently within a predetermined timeof first exposure to the electrolytic fluid to form pieces or particlessmall enough that they do not impede the flow of production fluidsthrough channel 214 of flow control device 206 (shown in FIG. 2) oropening 216 (shown in FIG. 2). The electrolytic fluid may already bepresent within annulus 212 (shown in FIG. 2) during operation ofwellbore 114 (shown in FIG. 1) or may be injected into annulus 212 totrigger a electrochemical reaction that causes plug 208 to degrade. Asanother example, plug 208 may be coated with a material that degradeswhen exposed to a wellbore fluid. A wellbore fluid may be circulatedaround the plug 208 in order to degrade the coating. Examples ofdegradable coatings include EPDM that degrades in crude oil, paint orplastics that degrades in xylene, or PGA or PLA that degrades in water.

Plug 208 may include a coating to temporarily protect the metal or alloyfrom exposure to the corrosive, acidic, or electrolytic fluid. As anexample, plug 208 may be coated with a material that softens or meltswhen a threshold temperature is reached in annulus 212 (shown in FIG.2). After the coating softens or melts, the surface of plug 208 may beexposed to the corrosive, acidic, or electrolytic fluid circulating inannulus 212 (shown in FIG. 2). As another example, plug 208 may becoated with a material that fractures when exposed to a thresholdpressure. The threshold pressure may be a pressure greater than apressure that occurs during operation of wellbore 114 (shown in FIG. 1).The pressure in wellbore 114 (shown in FIG. 1) or annulus 212 (shown inFIG. 2) may be manipulated such that it exceeds the threshold pressure,causing the coating to fracture. When the coating fractures, the surfaceof plug 208 may be exposed to the corrosive, acidic, or electrolyticfluid circulating in annulus 212 (shown in FIG. 2). As yet anotherexample, plug 208 may be coated with a material that erodes when exposedto a particle laden fluid. When the coating erodes, the surface of plug208 may be exposed to the corrosive, acidic, or electrolytic fluidcirculating in annulus 212 (shown in FIG. 2). Exemplary coatings may beselected from a metallic, ceramic, or polymeric material, andcombinations thereof. The coating may have low reactivity with thecorrosive, acidic, or electrolytic fluid present in annulus 212 (shownin FIG. 2), such that it protects plug 208 from degradation until thecoating is compromised allowing the corrosive, acidic, or electrolyticfluid to contact the metal or alloy.

FIG. 6B is a cross-sectional view of a degradable plug including a shelland a core disposed within the shell and formed of a degradablecomposition that is reactive under defined conditions. Plug 208 mayinclude core 604 disposed within channel 606 extending through shell608. Core 604 may be removed from shell 606 using a chemical reactionthat causes core 604 to degrade. Plug 208 also may include socket 602which may be configured to engage with a tool to permit plug 208 to bepositioned within or extracted from downhole assembly 200 (shown in FIG.2). Socket 602 may be open to channel 606 such that, when core 604 isremoved from shell 608, fluid may flow through plug 208 via socket 602and channel 606.

Core 604 may be formed of a degradable composition including a metal oralloy that is reactive under defined conditions. The composition of core604 may be selected such that core 604 begins to degrade within apredetermined time of first exposure to a corrosive or acidic fluid dueto reaction of the metal or alloy from which core 604 is formed with thecorrosive or acidic fluid. Additionally, the composition of plug 208 maybe selected such that the degradation of plug 208 accelerates withincreasing salinity or with decreasing pH of the corrosive or acidicfluid. The composition of core 604 may be selected such that core 604degrades sufficiently to form pieces or particles small enough that theydo not impede the flow of production fluids through shell 608. Thecorrosive or acidic fluid may already be present within annulus 212(shown in FIG. 2) during operation of wellbore 114 (shown in FIG. 1) ormay be injected into annulus 212 to trigger a chemical reaction thatcauses core 604 to degrade. Additionally, the fluid may be introduced aspart of the wellbore cleanup procedures. Examples of corrosive or acidicfluids include organic acids and inorganic acids, such as hydrochloricacid, acetic acid, citric acid, carbonic acid, lactic acid, glycolicacid, and hydrofluoric acid. Exemplary compositions from which core 604may be formed include compositions in which the metal or alloy isselected from one of calcium, magnesium, aluminum, and combinationsthereof. The composition of core 604 may be formed from a solutionprocess, from a powder metallurgy process, or from a nanomatrixcomposite. Additionally or alternatively, the composition of core 604may be cast, extruded, or forged. The composition of core 604 may alsobe heat treated or annealed.

Core 604 may also be formed from the metal or alloy imbedded with smallparticles (e.g., particulates, powders, flakes, fibers, and the like) ofa non-reactive material. The non-reactive material may be selected suchthat it remains structurally intact even when exposed to the corrosiveor acidic fluid for a duration of time sufficient to degrade the metalor alloy into pieces or particles small enough that they do not impedethe flow of production fluids through plug 208. When the metal or alloydegrades, the small particles of the non-reactive material may remain.The particle size of the non-reactive material may be selected such thatthe particles are small enough that they do not impede the flow ofproduction fluids through plug 208. The non-reactive material may beselected from one of lithium, bismuth, calcium, magnesium, and aluminum(including aluminum alloys) if not already selected as the reactivemetal or alloy, and combinations thereof.

Core 604 may also be formed from the metal or alloy imbedded with smallparticles (e.g., particulates, powders, flakes, fibers, and the like) toform a galvanic cell. The composition of the particles may be selectedsuch that the metal from which the particles are formed has a differentgalvanic potential than the metal or alloy in which the particles areimbedded. Contact between the particles and the metal or alloy in whichthey are imbedded may trigger microgalvanic corrosion that causes core604 to degrade. Exemplary compositions from which the particles may beformed include iron, steel, aluminum alloy, zinc, magnesium, graphite,nickel, copper, carbon, tungsten, and combinations thereof.

Core 604 may also be formed from an anodic material imbedded with smallparticles of cathodic material. The anodic and cathodic materials may beselected such that core 604 begins to degrade upon exposure to a brinefluid, which may also be referred to as an electrolytic fluid, due to anelectrochemical reaction that causes the plug to corrode. Brine fluidsmay include fluids containing NaCl, KCl, and other salts. Exemplarycompositions from which the anodic material may be formed include one ofmagnesium, aluminum, and combinations thereof. Exemplary compositionsfrom which the cathodic material may be formed include one of iron,nickel, copper, graphite, tungsten, and combinations thereof. The anodicand cathodic materials may be selected such that core 604 is degradedsufficiently within a predetermined time of first exposure to theelectrolytic fluid to form pieces or particles small enough that they donot impede the flow of production fluids through plug 208. Theelectrolytic fluid may already be present within annulus 212 (shown inFIG. 2) during operation of wellbore 114 (shown in FIG. 1) or may beinjected into annulus 212 to trigger a electrochemical reaction thatcauses core 604 to degrade.

Core 604 may include a coating to temporarily protect the metal or alloyfrom exposure to the corrosive, acidic, or electrolytic fluid. As anexample, core 604 may be coated with a material that softens or meltswhen a threshold temperature is reached in annulus 212 (shown in FIG.2). After the coating softens or melts, the surface of core 604 may beexposed to the corrosive, acidic, or electrolytic fluid circulating inannulus 212 (shown in FIG. 2). As another example, core 604 may becoated with a material that fractures when exposed to a thresholdpressure. The threshold pressure may be a pressure greater than apressure that occurs during operation of wellbore 114 (shown in FIG. 1).The pressure in wellbore 114 (shown in FIG. 1) or annulus 212 (shown inFIG. 2) may be manipulated such that it exceeds the threshold pressure,causing the coating to fracture. When the coating fractures, the surfaceof core 604 may be exposed to the corrosive, acidic, or electrolyticfluid circulating in annulus 212 (shown in FIG. 2). As yet anotherexample, core 604 may be coated with a material that erodes when exposedto a particle laden fluid. When the coating erodes, the surface of core604 may be exposed to the corrosive, acidic, or electrolytic fluidcirculating in annulus 212 (shown in FIG. 2). Exemplary coatings may beselected from a metallic, ceramic, or polymeric material, andcombinations thereof. The coating may have low reactivity with thecorrosive or acidic fluid present in annulus 212 (shown in FIG. 2), suchthat it protects core 604 from degradation until the coating iscompromised allowing the corrosive, acidic, or electrolytic to contactthe metal or alloy. As another example, core 604 may be coated with amaterial that degrades when exposed to a wellbore fluid. A wellborefluid may be circulated around core 604 in order to degrade the coating.Examples of degradable coatings include EPDM that degrades in crude oil,paint or plastics that degrades in xylene, or PGA or PLA that degradesin water.

Shell 608 may be formed of a non-reactive material. The non-reactivematerial may be selected such that it remains structurally intact evenwhen exposed to the corrosive or acidic fluid for a duration of timesufficient to degrade the metal or alloy from which core 604 is formedinto pieces or particles small enough that they do not impede the flowof production fluids through plug 208.

FIG. 6C is a cross-sectional view of a degradable plug including ashell, a core disposed within the shell and formed of a degradablecomposition that is reactive under defined conditions, and a rupturedisk. Plug 208 may include socket 602 which may be configured to engagewith a tool to permit plug 208 to be positioned within or extracted fromdownhole assembly 200 (shown in FIG. 2). Plug 208 may also include core604 disposed within channel 606 extending through shell 608. Asdiscussed above with respect to FIG. 6B, core 604 may be removed fromshell 610 using a chemical reaction that causes core 604 to degrade.Socket 602 may be open to channel 606 such that, when core 604 isremoved from shell 608, fluid may flow through plug 208 via socket 602and channel 606.

Plug 208 may further include rupture disk 618 that temporarily protectscore 604 from degradation until the rupture disk is compromised allowingthe corrosive or acidic fluid to contact the metal or alloy. Rupturedisk 618 may be formed of a material that fractures when exposed to athreshold pressure. The threshold pressure may be a pressure greaterthan a pressure that occurs during operation of wellbore 114 (shown inFIG. 1). The pressure in wellbore 114 (shown in FIG. 1) or annulus 212(shown in FIG. 2) may be manipulated such that it exceeds the thresholdpressure, causing rupture disk 618 to fracture. When rupture disk 618fractures, the surface of core 604 may be exposed to the brine fluid,corrosive fluid, or acidic fluid circulating in annulus 212 (shown inFIG. 2). As discussed above with respect to FIG. 6B, exposure to thebrine fluid, corrosive fluid, or acidic fluid may trigger a chemicalreaction or galvanic reaction that causes core 604 to degrade.

As discussed above with respect to FIG. 6B, shell 608 may be formed of anon-reactive material that remains structurally intact even when exposedto the corrosive or acidic fluid for a duration of time sufficient todegrade core 604 is formed into pieces or particles small enough thatthey do not impede the flow of production fluids through plug 208.

FIG. 6D is a cross-sectional view of a degradable plug including a coreformed of a degradable composition that is reactive under definedconditions and disposed within a shell including a diffusion channel.Plug 208 also may include socket 602 which may be configured to engagewith a tool to permit plug 208 to be positioned within or extracted fromdownhole assembly 200 (shown in FIG. 2). Plug 208 may also include core604 disposed within channel 614 extending axially through a portion ofshell 610. As discussed above with respect to FIG. 6B, core 604 may beremoved from shell 610 using a chemical reaction that causes core 604 todegrade.

Shell 610 may include diffusion channel 612 extending radially throughshell 610. When core 604 is removed from shell 610, fluid may flowthrough plug 208 via channel 614 and diffusion channel 612. Surface 616of shell 610 may act as a diffuser, deflecting fluids flowing throughchannel 614 into diffusion channel 612. Shell 610 may be formed of anon-reactive material. The non-reactive material may be selected suchthat it remains structurally intact even when exposed to the corrosiveor acidic fluid for a duration of time sufficient to degrade core 604into pieces or particles small enough that they do not impede the flowof production fluids through plug 208.

Although not illustrated in FIG. 6D, shell 610 may also include rupturedisk 618 (shown in FIG. 6C). As discussed with respect to FIG. 6C,rupture disk 618 may temporarily protect core 604 from degradation untilthe rupture disk is compromised allowing the corrosive or acidic fluidto contact the metal or alloy.

FIG. 7 illustrates a method of temporarily preventing the flow of fluidsinto or out of a production string. Method 700 may begin, and at step710, a plug may be positioned within a downhole assembly to temporarilyblock the flow of production fluids into a production string orinjection fluids out of the production string. As discussed above withrespect to FIG. 2, the downhole assembly may include a screen and ashroud, which may be coupled to and disposed downhole from the screen.Both the screen and the shroud may be coupled to and disposed around thecircumference of tubing coupled to the production string such that anannulus is formed between the inner surfaces of the screen and shroudand the outer surface of the tubing. The downhole assembly may alsoinclude a flow control device disposed within the annulus. The plug maybe positioned in the portion of the annulus downhole from the flowcontrol device.

In some embodiments, the plug may be positioned in-line with andadjacent to the flow control device, as shown in FIG. 2. In otherembodiments, the plug may be positioned in-line with and axiallydisplaced from the flow control device, as shown in FIG. 3. In stillother embodiments, the plug may positioned in an opening in the tubing,as shown in FIG. 4. As discussed above with respect to FIGS. 5A-5E, theplug may engage shroud and the tubing or the opening to form a fluid andpressure tight seal. Production fluids circulating in the wellbore mayenter the downhole assembly by flowing through the screen and into theannulus, but as discussed above with respect to FIGS. 2-4, the flow ofproduction fluids from the annulus into the tubing and the productionstring may be temporarily blocked by the plug. Similarly, injectionfluids circulating in the production string may be temporarily blockedfrom flowing into the formation by the plug.

The plug may be positioned within the downhole assembly before thedownhole assembly is positioned in the wellbore. Alternatively, the plugmay be positioned within the downhole assembly after the downholeassembly is positioned in the wellbore. As discussed above with respectto FIG. 2, the downhole assembly may include a port, which may beremoved to permit access to the portion of the annulus downhole from theflow control device. When the port has been removed, the plug may bepositioned within the downhole assembly.

At step 720, the plug (or the core of the plug) may be removed in orderto permit the flow of fluids into or out of the production string. Asdiscussed above with respect to FIGS. 6A-6D, the plug (or the core ofthe plug) may be removed by a chemical or electro-chemical reaction thatcauses the plug (or the core) to degrade. Once the chemical reaction hasbeen triggered, the reaction may continue until the plug (or the core)breaks down into pieces or dissolves into particles small enough thatthey do not impede the flow of production fluids. For example, where theentire plug degrades, the reaction may continue until the plug breaksdown into pieces or dissolves into particles small enough that they donot impede the flow of production fluids through the flow control deviceor the opening. Where only the core of the plug degrades, the reactionmay continue until the core breaks down into pieces or dissolves intoparticles small enough that they do not impede the flow of productionfluids through the flow control device, the opening, or the plug. Whenthe plug (or the core) has degraded to this point, fluids may flow intoand out of the production string.

At step 730, the flow of fluids into and out of the production stringmay be permitted. As discussed above with respect to step 710,production fluids circulating in the wellbore may enter the downholeassembly by flowing through a screen and into the annulus. Productionfluids circulating in the annulus may flow through a flow control devicedisposed in the annulus and into the portion of the annulus downholefrom flow the control device. From there, the production fluids may flowthrough an opening=formed in a sidewall of tubing coupled to theproduction string and into the production string. Similarly, injectionfluids circulating in the production string may flow into the annulusthrough the opening formed in the sidewall of the tubing. From there,the injection fluids may flow through the flow control device disposedin the annulus and into the formation.

At step 740, a determination may be made regarding whether totemporarily prevent the flow of fluids into or out of the productionstring. If it is determined to temporarily prevent the flow of fluidsinto the production string, the method may return to step 710. If it isdetermined not to temporarily prevent the flow of fluids into theproduction string, the method may end.

Modifications, additions, or omissions may be made to method 700 withoutdeparting from the scope of the present disclosure. For example, theorder of the steps may be performed in a different manner than thatdescribed and some steps may be performed at the same time.Additionally, each individual step may include additional steps withoutdeparting from the scope of the present disclosure.

Embodiments disclosed herein include:

A. A downhole assembly that includes a tube disposed in a wellbore, ashroud coupled to and disposed around the circumference of the tube toform an annulus between an inner surface of the shroud and an outersurface of the tube, a flow control device disposed in the annulus, anda degradable plug disposed in the annulus and positioned to preventfluid flow between the annulus and the tube.

B. A well system that includes a production string, and a downholeassembly coupled to and disposed downhole from the production string.The downhole assembly includes a tube, a shroud coupled to and disposedaround the circumference of the tube to form an annulus between an innersurface of the shroud and an outer surface of the tube, a flow controldevice disposed in the annulus, and a degradable plug disposed in theannulus and positioned to prevent fluid flow between the annulus and thetube.

C. A method of temporarily preventing fluid flow between a productionstring and a wellbore that includes positioning a degradable plug in awellbore such that the plug prevents fluid flow between a productionstring and a wellbore, and triggering a chemical reaction that causesthe degradable plug to degrade to a point where fluid flow between theproduction string and the wellbore is permitted.

Each of embodiments A, B, and C may have one or more of the followingadditional elements in any combination: Element 1: the downhole assemblyfurther includes a screen coupled to and disposed uphole from the shroudand coupled to and disposed around the circumference of the tube suchthat an annulus is formed between an inner surface of the screen and theouter surface of the tube. Element 2: wherein the degradable plug ispositioned in-line with and adjacent to the flow control device. Element3: wherein the degradable plug is positioned in-line with and axiallydisplaced from the flow control device. Element 4: wherein thedegradable plug is engaged with the shroud and the tube to form a fluidand pressure tight seal. Element 5: wherein the degradable plug ispositioned in an opening formed in a sidewall of the tube, and engagedwith the tube to form a fluid and pressure tight seal and prevent fluidflow between the annulus and the tube. Element 6: wherein the degradableplug is formed of a composition that degrades within the annulus withina predetermined time of exposure to a particular fluid. Element 7:wherein the degradable plug includes a degradable plug formed of acomposition that degrades within the annulus within a predetermined timeof exposure to a particular fluid, and a coating formed around thedegradable plug that temporarily protects the degradable plug fromexposure to the particular fluid. Element 8: wherein the degradable plugcomprises a first composition imbedded with particles of a secondcomposition to form a galvanic cell. Element 9: wherein the degradableplug includes a shell including a channel extending there through, and adegradable core disposed within the channel and formed of a compositionthat degrades within the annulus within a predetermined time of exposureto a particular fluid. Element 10: wherein the degradable plug includesa shell including a channel extending there through, a degradable coredisposed within the shell and formed of a composition that degradeswithin the annulus within a predetermined time of first exposure to aparticular fluid, and a rupture disk that temporarily protects thedegradable plug from exposure to the particular fluid, the rupture diskformed of a material that fractures when exposed to a thresholdpressure. Element 11: wherein the degradable plug includes a shellincluding a first channel extending radially there through, and a secondchannel extending axially from an outer surface of the shell to thefirst channel, and a degradable core disposed within the second channeland formed of a composition that degrades within the annulus within apredetermined time of exposure to a particular fluid. Element 12:wherein the degradable plug includes a rupture disk that temporarilyprotects the degradable core from exposure to the particular fluid, therupture disk formed of a material that fractures when exposed to athreshold pressure.

Element 13: wherein the degradable plug is positioned in fluidcommunication with a flow control device. Element 14: wherein thechemical reaction is triggered by exposure of the degradable plug to aparticular fluid for an amount of time exceeding a threshold time.Element 15: wherein triggering the chemical reaction comprises removinga protective coating formed around the degradable plug to expose thedegradable plug to a particular fluid. Element 16: wherein removing theprotective coating comprises exposing the degradable plug to a thresholdtemperature that causes the protective coating to melt. Element 17:wherein removing the protective coating comprises exposing thedegradable plug to a threshold pressure that causes the protectivecoating to fracture. Element 18: wherein the degradable plug degradesinto particles small enough that they do not impede fluid flow. Element19: wherein the chemical reaction causes a core of the degradable plugto degrade to a point where flow of fluids through the degradable plugis permitted. Element 20: wherein triggering the chemical reactioncomprises rupturing a rupture disk to expose a core of the degradableplug to a particular fluid for an amount of time exceeding a thresholdtime.

Therefore, the disclosed systems and methods are well adapted to attainthe ends and advantages mentioned as well as those that are inherenttherein. The particular embodiments disclosed above are illustrativeonly, as the teachings of the present disclosure may be modified andpracticed in different but equivalent manners apparent to those skilledin the art having the benefit of the teachings herein. Furthermore, nolimitations are intended to the details of construction or design hereinshown, other than as described in the claims below. It is thereforeevident that the particular illustrative embodiments disclosed above maybe altered, combined, or modified and all such variations are consideredwithin the scope of the present disclosure. The systems and methodsillustratively disclosed herein may suitably be practiced in the absenceof any element that is not specifically disclosed herein and/or anyoptional element disclosed herein.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the following claims.

1. A downhole assembly, comprising: a tube disposed in a wellbore; ashroud coupled to and disposed around the circumference of the tube toform an annulus between an inner surface of the shroud and an outersurface of the tube; a flow control device disposed in the annulus; anda degradable plug disposed in the annulus and positioned to preventfluid flow between the annulus and the tube.
 2. The downhole assembly ofclaim 1, further comprising a screen coupled to and disposed uphole fromthe shroud and coupled to and disposed around the circumference of thetube such that an annulus is formed between an inner surface of thescreen and the outer surface of the tube.
 3. The downhole assembly ofclaim 1, wherein the degradable plug is positioned in-line with andadjacent to or axially displaced from the flow control device. 4.(canceled)
 5. The downhole assembly of claim 1, wherein the degradableplug is engaged with the shroud and the tube to form a fluid andpressure tight seal.
 6. The downhole assembly of claim 1, wherein thedegradable plug is positioned in an opening formed in a sidewall of thetube and engaged with the tube to form a fluid and pressure tight sealto prevent fluid flow between the annulus and the tube.
 7. The downholeassembly of claim 1, wherein the degradable plug is formed of acomposition that degrades within the annulus within a predetermined timeof exposure to a particular fluid.
 8. The downhole assembly of claim 7,wherein the degradable plug comprises a coating formed around thedegradable plug that temporarily protects the degradable plug fromexposure to the particular fluid.
 9. The downhole assembly of claim 1,wherein the degradable plug comprises a first composition imbedded withparticles of a second composition to form a galvanic cell.
 10. Thedownhole assembly of claim 1, wherein the degradable plug comprises: ashell including a channel extending there through; and a degradable coredisposed within the channel and formed of a composition that degradeswithin the annulus within a predetermined time of exposure to aparticular fluid.
 11. The downhole assembly of claim 1, wherein thedegradable plug comprises: a shell including a channel extending therethrough; a degradable core disposed within the shell and formed of acomposition that degrades within the annulus within a predetermined timeof first exposure to a particular fluid; and a rupture disk thattemporarily protects the degradable plug from exposure to the particularfluid, the rupture disk formed of a material that fractures when exposedto a threshold pressure.
 12. The downhole assembly of claim 1, whereinthe degradable plug comprises: a shell including: a first channelextending radially there through; and a second channel extending axiallyfrom an outer surface of the shell to the first channel; and adegradable core disposed within the second channel and formed of acomposition that degrades within the annulus within a predetermined timeof exposure to a particular fluid.
 13. (canceled)
 14. A well systemcomprising: a production string; and a downhole assembly coupled to anddisposed downhole from the production string, the downhole assemblycomprising: a tube; a shroud coupled to and disposed around thecircumference of the tube to form an annulus between an inner surface ofthe shroud and an outer surface of the tube; a flow control devicedisposed in the annulus; and a degradable plug disposed in the annulusand positioned to prevent fluid flow between the annulus and the tube.15. The well system of claim 14, wherein the downhole assembly furthercomprises a screen coupled to and disposed uphole from the shroud andcoupled to and disposed around the circumference of the tube such thatan annulus is formed between an inner surface of the screen and theouter surface of the tube.
 16. The well system of claim 14, wherein thedegradable plug is positioned in-line with and adjacent to or axiallydisplaced from the flow control device.
 17. (canceled)
 18. (canceled)19. The well system of claim 14, wherein the degradable plug ispositioned in an opening formed in a sidewall of the tube and engagedwith the tube to form a fluid and pressure tight seal to prevent fluidflow between the annulus and the tube.
 20. The well system of claim 14,wherein the degradable plug is formed of a composition that degradeswithin the annulus within a predetermined time of exposure to aparticular fluid.
 21. The well system of claim 20, wherein thedegradable plug comprises a coating formed around the degradable plugthat temporarily protects the degradable plug from exposure to theparticular fluid.
 22. (canceled)
 23. The well system of claim 14,wherein the degradable plug comprises: a shell including a channelextending there through; and a degradable core disposed within thechannel and formed of a composition that degrades within the annuluswithin a predetermined time of exposure to a particular fluid.
 24. Thewell system of claim 14, wherein the degradable plug comprises: a shellincluding a channel extending there through; a degradable core disposedwithin the shell and formed of a composition that degrades within theannulus within a predetermined time of first exposure to a particularfluid; and a rupture disk that temporarily protects the degradable plugfrom exposure to the particular fluid, the rupture disk formed of amaterial that fractures when exposed to a threshold pressure.
 25. Thewell system of claim 14, wherein the degradable plug comprises: a shellincluding: a first channel extending radially there through; and asecond channel extending axially from an outer surface of the shell tothe first channel; and a degradable core disposed within the secondchannel and formed of a composition that degrades within the annuluswithin a predetermined time of exposure to a particular fluid. 26.(canceled)
 27. A method of temporarily preventing fluid flow between aproduction string and a wellbore, comprising: positioning a degradableplug in a wellbore such that the plug prevents fluid flow between aproduction string and a wellbore; and triggering a chemical reactionthat causes the degradable plug to degrade to a point where fluid flowbetween the production string and the wellbore is permitted.
 28. Themethod of claim 27, wherein the degradable plug is positioned in fluidcommunication with a flow control device.
 29. The method of claim 28,wherein the degradable plug is positioned in-line with and adjacent toor axially displaced from the flow control device.
 29. (canceled) 30.The method of claim 27, wherein the chemical reaction is triggered byexposure of the degradable plug to a particular fluid for an amount oftime exceeding a threshold time.
 31. The method of claim 27, whereintriggering the chemical reaction comprises removing a protective coatingformed around the degradable plug to expose the degradable plug to aparticular fluid.
 32. The method of claim 31, wherein removing theprotective coating comprises exposing the degradable plug to a thresholdtemperature that causes the protective coating to melt or a thresholdpressure that causes the protective coating to fracture.
 33. (canceled)34. The method of claim 27, wherein the degradable plug degrades intoparticles small enough such that the particles do not impede fluid flow.35. (canceled)
 36. The method of claim 27, wherein triggering thechemical reaction comprises rupturing a rupture disk to expose a core ofthe degradable plug to a particular fluid for an amount of timeexceeding a threshold time.